1. Field of the Invention
The present invention relates to a system for reading a radiation image from an image signal of a subject. In particular, the present invention relates to a sampling technology of an image signal.
2. Description of Related Art
So far, a radiation image such as an X-ray image or the like has been widely used in a medical field for diagnosing a medical condition. In particular, a radiation image according to the Intensifying screen-Film system still remains being used in a world-wide medical field as an imaging system having both a high reliability and an excellent cost performance, as a result of efforts for enhancing a sensitivity and an image quality in a long history. On the other hand, these days, a Computed Radiography (CR) using a photostimulable phosphor sheet is in commercialization and a sensitivity thereof and an image quality thereof have been enhanced. Furthermore, a Flat Panel Detector (FPD) has also been introduced. Therefore, nowadays there are more choices of a system instead of the conventional Intensifying screen-Film system.
Meanwhile, in conjunction with developments of a radiation detector such as a CR, an FPD or the like, opportunities of generating a magnified image for looking at a more detailed structure of the subject are increasing (for example, see JP-Tokukai-2001-311701A and JP-Tokukai-2003-180670A). In the magnified image generation, while it is possible to look at a detailed structure of the subject as mentioned, there is in general a problem of an image deterioration because blur is generated on a subject image according to an image generation condition, since a radiation source is not an ideal point source. In the arts disclosed in JP-Tokukai-2001-311701A and JP-Tokukai-2003-180670A, such a problem is solved by using a phase contrast or the like, and thereby it is possible to obtain a radiation image having an excellent sharpness.
By the way, regardless of a type of the radiation detector such as a CR, an FPD or the like, it is always necessary to have a step of “sampling” in order to obtain a digital signal from an image signal (analog signal) obtained by the radiation detector. According to a known sampling principle, a phenomenon called “aliasing” occurs unless the image signal obtained by the radiation detector is sampled by a sampling frequency being larger than a frequency two times as much as the frequency of the image signal prior to the sampling. When “aliasing” occurs, a false image is generated or a noise is increased.
When a CR is applied as the radiation detector, it is possible to obtain an image signal by following a time series along with a scanning direction of a stimulating light. Therefore, it is possible to prevent the aliasing from occurring in the scanning direction by providing a low pass filter called an anti-aliasing filter. However, since it is not possible to provide such a filter in a sub-scanning direction, which is perpendicular to the scanning direction of the stimulating light, it is not possible to avoid the occurrence of the aliasing in the sub-scanning direction.
On the contrary, when an FPD is applied as the radiation detector, principally or due to a structural reason, it is difficult to place the anti-aliasing filter, and therefore the aliasing always occurs. Further, in this case, the larger a value of a Modulation Transfer Function (MTF) of the detector for obtaining a radiation image having a high sharpness is set, the larger influence the aliasing provides.
Under such circumstances, when an image signal is sampled, it is necessary to perform an optimal sampling corresponding to a type of the radiation detector while the occurrence of the aliasing is considered. Moreover, since it is considered that opportunities of the above-mentioned magnified image generation keep increasing, it is also necessary to perform an optimal sampling in view of the above-mentioned magnified image generation.